Many types of telecommunication systems have been developed to exchange different kinds of information between specialized terminal devices. For example, separate telecommunication systems are often used for exchanging voice, data or video signals. Rather than provide separate telecommunication systems for each of these services, the use of an Integrated Services Digital Network (ISDN) is gaining popularity among users who need flexible access to different types of communication services. Several standards and recommendations pertaining to ISDN have been developed in order to specify a uniform system for encoding, transmitting, and switching signals in a single network among various kinds of terminal devices. These standards and recommendations include BellCore TR41449, AT&T 5ESS Customer National ISDN PRI Specification, ANSI TI.607, CCITT Q.931, and BellCore Guideline SR-NWT-2343 for National ISDN PRI CRF.
An ISDN interface provides the user with access to a number of full duplex, time-division multiplexed (TDM) channels. One of these channels can be used for call control messages that pertain to the remaining channels. In the T1 Primary Rate Interface standard, for example, 24 full duplex channels are multiplexed onto a communication link between the customer premises and the ISDN. Twenty-three of the channels, designated B-channels, can be used for communication between terminal devices. Each full duplex channel provides a virtual "4-wire" connection between the terminal equipment using the channel. The 24th channel, designated the D-channel, can be used for call control signals pertaining to the transmission of information on the B-channels and to the status of connections established between terminal devices via the B-channels.
In the transmission of signals involving 2-wire loop devices, such as standard telephones, it is necessary to provide an interface device between the 2-wire loop device and a 4-wire full duplex transmission link. The interface device, or hybrid, separates the inbound and outbound signals, which are combined on the 2-wire loop, into separate send and receive paths on the 4-wire link. Because hybrids cannot provide absolute separation of the send and receive signals, some residual coupling between the send and receive signals remains. This coupling can be perceived by a telephone user as echo.
In order to eliminate echo, devices known as echo cancelers have been developed. Briefly, an echo canceler monitors the received signal, computes an expected echo signal on the basis of the received signal and the impulse response of the local loop, and then subtracts the expected echo signal from the transmitted signal. In digital transmission systems, the function of echo canceling is accomplished using digital signal processors which are programmed to manipulate the data encoded in the transmitted signal that represents an audio signal. Such echo cancelers function acceptably on voice communication calls, but it is undesirable to employ echo cancelers on data communication calls which rely upon unaltered transmission of digital signals.
Because it is desirable to employ echo cancellation on some calls and not on other calls, some users have arranged their ISDN interfaces so that echo cancellation is always applied to certain channels and never applied to other channels. Then, the use of each ISDN channel is restricted to the types of calls that are compatible with the predetermined presence or absence of echo canceling on the channel. This solution is inconvenient because it reduces one of the primary benefits of ISDN, i.e. the user's flexibility of access to ISDN channels. It would be desirable to provide a system in which echo canceling on each ISDN channel is automatically enabled or disabled depending upon the type of call being made on each channel.